In the sector of skin-prints detection, devices have been developed comprising a single detection surface (or “platen”) associated to which is a single sensor capable of operating both over a “large” area, for detecting, for example, the palmprint of a hand, and over a “medium” or “small” area, for detecting, for example, the fingerprint of a finger that is rotated or rolled on the detection surface or else the fingerprint of just the tip of a finger.
For instance, known from the document No. EP 2 400 432 A1 is a detection device comprising a detection surface, a sensor for generating a sequence of images of a moving object, such as a finger that is rolled on the detection surface, and processing means for processing the sequence of images generated by the sensor which are configured for identifying, in the object that is rolled, a front edge in the rolling movement, with the sensor configured for generating the sequence of images with a detection window that follows the aforesaid front edge in the rolling movement.
To carry out detection properly, the finger may need to be positioned initially with a certain degree surface, for example, on one side (on the left or on the right) of the detection surface.
This may be required, for example, in order to leave space free for the rolling movement in a direction opposite to the starting one, preventing the skin print from being “clipped” on the opposite edge of the fixed area thus limiting the rolling movement of the object in this direction.
The boundaries of these fixed areas of positioning, that form part of the entire detection surface, may be difficult to distinguish in a visible way directly on the detection surface, which is constituted, for example, by a (semi)transparent glass screen.
A practical way of delimiting these fixed areas may be that of using markers. However, this solution can create some drawbacks in execution of a precise initial positioning, for example, of a finger along the perimeter of a predefined rolling area (above all for objects with a long rolling path such as an entire finger that is being rolled). This may lead to having to make various attempts of initial positioning of the finger (or, in general, of the part of hand concerned); i.e., it may lead to a number of “positioning and checking” operations, before actual start of the rolling step.
Furthermore, since fingerprint scanning may be used in applications of law enforcement (for example, in police stations), where the subjects may not be particularly co-operative or may even put up resistance in regard to the taking of fingerprints, the need to achieve a precise initial positioning of the finger may constitute a non-negligible problem.